SpringMVC

[wittyResry]

HttpServletBean

• 保留了initServletBean的留给继承者实现实现

  public final void init() throws ServletException {
      if (logger.isDebugEnabled()) {
          logger.debug("Initializing servlet '" + getServletName() + "'");
      }
  
      // Set bean properties from init parameters.
      try {
          PropertyValues pvs = new ServletConfigPropertyValues(getServletConfig(), this.requiredProperties);
          BeanWrapper bw = PropertyAccessorFactory.forBeanPropertyAccess(this);
          ResourceLoader resourceLoader = new ServletContextResourceLoader(getServletContext());
          bw.registerCustomEditor(Resource.class, new ResourceEditor(resourceLoader, getEnvironment()));
          initBeanWrapper(bw);
          bw.setPropertyValues(pvs, true);
      }
      catch (BeansException ex) {
          logger.error("Failed to set bean properties on servlet '" + getServletName() + "'", ex);
          throw ex;
      }
  
      // Let subclasses do whatever initialization they like.
      initServletBean();
  
      if (logger.isDebugEnabled()) {
          logger.debug("Servlet '" + getServletName() + "' configured successfully");
      }
  }

[wittyResry]

FrameworkServlet

• 1.重写了HttpServletBean中的initServletBean方法
• 2. 重写 HttpServlet 的 service() 方法

关于重写 initServletBean() 方法

• 首先initWebApplicationContext，那么什么是WebApplicationContext呢？

  protected void configureAndRefreshWebApplicationContext(ConfigurableWebApplicationContext wac) {
      if (ObjectUtils.identityToString(wac).equals(wac.getId())) {
          // The application context id is still set to its original default value
          // -> assign a more useful id based on available information
          if (this.contextId != null) {
              wac.setId(this.contextId);
          }
          else {
              // Generate default id...
              wac.setId(ConfigurableWebApplicationContext.APPLICATION_CONTEXT_ID_PREFIX +
                      ObjectUtils.getDisplayString(getServletContext().getContextPath()) + "/" + getServletName());
          }
      }
  
      wac.setServletContext(getServletContext());
      wac.setServletConfig(getServletConfig());
      wac.setNamespace(getNamespace());
      wac.addApplicationListener(new SourceFilteringListener(wac, new ContextRefreshListener()));
  
      // The wac environment's #initPropertySources will be called in any case when the context
      // is refreshed; do it eagerly here to ensure servlet property sources are in place for
      // use in any post-processing or initialization that occurs below prior to #refresh
      ConfigurableEnvironment env = wac.getEnvironment();
      if (env instanceof ConfigurableWebEnvironment) {
          ((ConfigurableWebEnvironment) env).initPropertySources(getServletContext(), getServletConfig());
      }
  
      postProcessWebApplicationContext(wac);
      applyInitializers(wac);
      wac.refresh();
  }

• WebApplicationContext接口继承了ApplicationContext，可通过getServletContext方法获取ServletContext

public interface WebApplicationContext extends ApplicationContext {
    ServletContext getServletContext();
}
public interface ApplicationContext extends EnvironmentCapable, ListableBeanFactory, HierarchicalBeanFactory,
        MessageSource, ApplicationEventPublisher, ResourcePatternResolver {
    String getId();
    String getApplicationName();
    String getDisplayName();
    long getStartupDate();
    ApplicationContext getParent();

    /**
     * Expose AutowireCapableBeanFactory functionality for this context.
     * <p>This is not typically used by application code, except for the purpose of
     * initializing bean instances that live outside of the application context,
     * applying the Spring bean lifecycle (fully or partly) to them.
     * <p>Alternatively, the internal BeanFactory exposed by the
     * {@link ConfigurableApplicationContext} interface offers access to the
     * {@link AutowireCapableBeanFactory} interface too. The present method mainly
     * serves as a convenient, specific facility on the ApplicationContext interface.
     * <p><b>NOTE: As of 4.2, this method will consistently throw IllegalStateException
     * after the application context has been closed.</b> In current Spring Framework
     * versions, only refreshable application contexts behave that way; as of 4.2,
     * all application context implementations will be required to comply.
     * @return the AutowireCapableBeanFactory for this context
     * @throws IllegalStateException if the context does not support the
     * {@link AutowireCapableBeanFactory} interface, or does not hold an
     * autowire-capable bean factory yet (e.g. if {@code refresh()} has
     * never been called), or if the context has been closed already
     * @see ConfigurableApplicationContext#refresh()
     * @see ConfigurableApplicationContext#getBeanFactory()
     */
    AutowireCapableBeanFactory getAutowireCapableBeanFactory() throws IllegalStateException;
}

• ServletContext是什么？

  WEB容器在启动时，它会为每个WEB应用程序都创建一个对应的ServletContext对象，它代表当前web应用。ServletConfig对象中维护了ServletContext对象的引用，开发人员在编写servlet时，可以通过ServletConfig.getServletContext方法获得ServletContext对象。

• 重写的方式如下：

  @Override
  protected final void doGet(HttpServletRequest request, HttpServletResponse response)
  throws ServletException, IOException {
  
  processRequest(request, response);
  }
  @Override
  protected final void doPost(HttpServletRequest request, HttpServletResponse response)
  throws ServletException, IOException {
  
  processRequest(request, response);
  }

• processRequest(request, response)处理逻辑：

  protected final void processRequest(HttpServletRequest request, HttpServletResponse response)
          throws ServletException, IOException {
      try {
          doService(request, response);
      }
  }

• 对于 doService(request, response)方法，交由子类DispatcherServlet完成：

  protected abstract void doService(HttpServletRequest request, HttpServletResponse response)
          throws Exception;

• 所以重点分析下DispatcherServlet的doService方法

[wittyResry]

DispatcherServlet

• DispatcherServlet是SpringMVC 的核心调度器，也是SpringMVC 的核心。
• FrameworkServlet 说过，该类主要留下了两个方法给子类实现：onRefresh() 方法和 doService()

onRefresh()

@Override
protected void onRefresh(ApplicationContext context) {
    initStrategies(context);
}
protected void initStrategies(ApplicationContext context) {
    initMultipartResolver(context);
    initLocaleResolver(context);
    initThemeResolver(context);
    initHandlerMappings(context);
    initHandlerAdapters(context);
    initHandlerExceptionResolvers(context);
    initRequestToViewNameTranslator(context);
    initViewResolvers(context);
    initFlashMapManager(context);
}

下面分析下initHandlerMappings(context)

Using a handler mapping you can map incoming web requests to appropriate handlers：当一个请求过来的时候，HandlerMapping 负责把处理该请求的 handler 找到， HandlerAdapter 则负责执行这个方法。 
The strategy interface HandlerAdapter takes the role of invoking handler methods selected by some HandlerMapping. HandlerAdapter负责执行HandlerMapping获取的handler方法
This allows for different types of controllers use different method invokation strategies. and URL mapping strategies are possible.
The first HandlerAdapter that supports the handler object (controller or method) is used to invoke the handler.
Spring MVC supports several types of controllers, including: annotation defined controllers (with @Controller), the WebFlow FlowExecutor etc.
AnnotationMethodHandlerAdapter

AnnotationMethodHandlerAdapter adapts HTTP requests to handler methods annotated with @RequestMapping. It introspects required input arguments for handlers, and interprets output values. This the HandlerAdapter configured by default.
The AnnotationMethodHandlerAdapter can be used with controller with multiple handler methods in a variety of powerful ways.

doService()

@Override
    protected void doService(HttpServletRequest request, HttpServletResponse response) throws Exception {

        try {
            doDispatch(request, response);
        }
}

    protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
        HttpServletRequest processedRequest = request;
        HandlerExecutionChain mappedHandler = null;
        boolean multipartRequestParsed = false;

        WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);

        try {
            ModelAndView mv = null;
            Exception dispatchException = null;

            try {
                // 先对请求进行封装。
                processedRequest = checkMultipart(request);
                multipartRequestParsed = (processedRequest != request);

                // Determine handler for the current request.
                mappedHandler = getHandler(processedRequest);
                if (mappedHandler == null || mappedHandler.getHandler() == null) {
                    noHandlerFound(processedRequest, response);
                    return;
                }
                //像之前说的，拿到处理该请求的 HandlerMapping。
                HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

                // Process last-modified header, if supported by the handler.
                String method = request.getMethod();
                boolean isGet = "GET".equals(method);
                if (isGet || "HEAD".equals(method)) {
                    long lastModified = ha.getLastModified(request, mappedHandler.getHandler());
                    if (logger.isDebugEnabled()) {
                        logger.debug("Last-Modified value for [" + getRequestUri(request) + "] is: " + lastModified);
                    }
                    if (new ServletWebRequest(request, response).checkNotModified(lastModified) && isGet) {
                        return;
                    }
                }

                if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                    return;
                }
                // Actually invoke the handler.
                //具体调用该方法处理这个请求，并且返回一个 ModelAndView 对象，该对象包含了视图和数据
                mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

                if (asyncManager.isConcurrentHandlingStarted()) {
                    return;
                }

                applyDefaultViewName(processedRequest, mv);
                /** 利用视图解析器对视图对象 mv 进行解析，并且 response 返回。*/
                mappedHandler.applyPostHandle(processedRequest, response, mv);
            }
            catch (Exception ex) {
                dispatchException = ex;
            }
            catch (Throwable err) {
                // As of 4.3, we're processing Errors thrown from handler methods as well,
                // making them available for @ExceptionHandler methods and other scenarios.
                dispatchException = new NestedServletException("Handler dispatch failed", err);
            }
            processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
        }
        catch (Exception ex) {
            triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
        }
        catch (Throwable err) {
            triggerAfterCompletion(processedRequest, response, mappedHandler,
                    new NestedServletException("Handler processing failed", err));
        }
        finally {
            if (asyncManager.isConcurrentHandlingStarted()) {
                // Instead of postHandle and afterCompletion
                if (mappedHandler != null) {
                    mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response);
                }
            }
            else {
                // Clean up any resources used by a multipart request.
                if (multipartRequestParsed) {
                    cleanupMultipart(processedRequest);
                }
            }
        }
    }

[wittyResry]

在Spring MVC里，如果把@ModelAttribute放在方法的注解上时，代表的是：该Controller的所有方法在调用前，先执行此@ModelAttribute方法。